[vsnet-grb-info 1845] GRB 050724: dust-scattered X-ray halo detected in SWIFT/XRT observation

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3685
SUBJECT: GRB 050724: dust-scattered X-ray halo detected in SWIFT/XRT observation
DATE:    05/07/26 22:11:58 GMT
FROM:    Pat Romano at OAB-Swift  <romano at merate.mi.astro.it>

P. Romano, A. Moretti (OAB), S. Vaughan (U. Leicester), L. A. Antonelli (OAR), S. Campana, G. Chincarini, S. Covino,D. Malesani, G. Tagliaferri (OAB), J. Osborne, R. Willingale, P.T. O'Brien, A. Levan, K. L. Page, M. R. Goad (U. Leicester), D. N. Burrows (PSU), report:

Analysis of the SWIFT XRT observation of GRB050724 in Photon Counting (PC) mode has revealed in the first orbit a diffuse X-ray halo centered around the afterglow location. The halo has the form of complete ring which increases in radius through the observation and reached the distance of ~100 arcsec during the first ~2000 sec after the trigger (T0). We can exclude it is due to instrumental effects. During the observation, which started ~343 seconds after the burst trigger time (PC mode), the observed halo follows the expected behaviour of a "light-echo" as X-rays are scattered by dust in our Galaxy. From the radial expansion of the halo we estimate a distance of 175 -/+ 50 pc to the dust (from the observer, 90% confidence interval).

GRB 050724 is in the direction (Galactic) l = 350.37, b = 15.10 degrees, in which the density of interstellar medium is quite high as testified by both the neutral Hydrogen column density nH=1.5E21 cm-2 and the optical extiction E(B-V)=0.61 (A_V=2.029, A_K=0.225). This line of sight includes the Scorpius Centaurus OB association at ~150-200 pc, consistent with the derived distance to the scattering medium.

The X-ray WT spectrum of GRB050724 is represented by a powerlaw with Photon index 1.93-/+0.05 in the [0.5-10] keV band (as reported in GCN 3669; Romano et al.). The scattered X-ray light has, as expected, a softer spectrum with Photon index 3.2-0.8+1.0 (first orbit).

More information is posted at: http://www.merate.mi.astro.it/~romano/grb050724/index.html